i) Field of the Invention
This invention relates to a method of sequentially developing nebulized samples of a liquid for analysis; the invention also relates to a method of clearing potentially interfering residual sample from a sample chamber in which nebulized samples are developed, to reduce residual sample in the chamber to a background level; still further the invention relates to a method of inductively coupled plasma analysis; and to a sample chamber for development of a nebulized sample of a liquid for analysis outside the chamber.
ii) Description of Prior Art
The ability to determine many elements over a wide concentration range has made inductively coupled plasma-atomic emission spectroscopy (ICP-AES) the workhorse of elemental analysis.
Samples for analysis are typically developed in a nebulized or aerosol form in a sample chamber and the nebulized sample is then transferred from the chamber to the analysis device or equipment.
Automation can substantially reduce the need for manual labor to run the nebulized samples and improve the potential throughput of the system; however, before a fresh sample can be analyzed the effects of the last sample must be removed. The time taken to reduce the signal from residual sample so that it will not compromise the measurement of the analyte signal of the subsequent sample to be analyzed is called "wash-out time".
A wash-out time sufficient to deal with the largest concentration difference that might arise between contiguous samples is used for most applications. In an environmental laboratory this could be as long as ten minutes, but generally a two minute wash-out is used after which data are reviewed and suspect samples rerun.
Polychromators and some ICP-MS (Mass Spectrometry) spectrometers can perform their analyses in approximately 10 seconds, while the wash-out time for commercial and even experimental nebulizer sample introduction generally ranges from 30 seconds to 4 minutes. Dobb and Jenke, Appl. Spectrosc. 1983, 37, 379, report that rigorous correction for the memory by allowing for sufficient sample washout (T.sub.0.01) increases analysis time per sample by a factor of 5. Clearly, a significant improvement in wash-out time could drastically affect the throughput of ICP based systems.
Many ways have been suggested to evaluate wash-out time. The evaluation criteria which have been suggested differ significantly and require that one apply considerable caution in comparing the results.
Wash-out time can be the time required for the signal to approach an arbitrarily selected value after aspirating a randomly selected sample solution. The word "arbitrary" is used to indicate that there is no fundamental relationship to dictate testing criteria.
A more precise definition of wash-out time proposed by R. M. Clifford et al, Anal. Chem. 1989, 61, 2777, requires reduction of the signal of residual sample to the 1% level, after an analyte solution is replaced with a blank solution (time of exchange). The actual moment to start timing is the instant of exchange, but some authors use the first sign of signal decay from the steady-state value. Measuring wash-out time from the time of exchange is more realistic in evaluating sample throughput. If the analyte level of the samples varies over several orders of magnitude, samples with relatively low levels that follow samples at very high levels will be biased high unless the wash-out time is increased considerably. Another definition of wash-out time uses a solution that is at least five orders of magnitude above the limit of detection for the particular element of interest. Typically a 1000 .mu.g/g pure stock solution is used and replaced with a blank. The wash-out time is measured from the time of exchange until a signal level equivalent to .+-.0.01 .mu.g/g of the original analyte signal level is reached. When the first definition (1%) is used to evaluate wash-out time, a time of 20 seconds to several minutes can be obtained, but when the latter definition is used, wash-out times of an order of magnitude longer are possible. For the comparison of two systems, a wash-out to 1% of the original signal level is useful, but, for analyses, any measurable trace of previous analyte must be removed.
It is clearly desirable to decrease the wash-out time to improve efficiency. It has been suggested that there are at least four major causes of long wash-out times in spray chambers which include dead volumes in the nebulizer tubing, adsorption and subsequent slow release of residual sample by the nebulizer tubing, size of the sample development chamber or spray chamber, dead volume in the spray chamber, entrainment of residual sample by gas issuing from the nebulizer and reaspiration of the entrained sample.
Thus far efforts to reduce wash-out time by approaches directed to these causes have not met with any significant success.